System and method for ammonia synthesis

ABSTRACT

Systems and methods are disclosed herein for synthesizing ammonia using nano-size metal or metal alloy catalyst particles. Hydrogen and nitrogen gases are passed through a system comprising, for example, a bed of magnetite supporting nano-size iron or iron alloy catalyst particles having an optional oxide layer that forms the catalyst.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/752,018, filed Mar. 31, 2010, which is a continuation-in-part of U.S.patent application Ser. No. 12/418,356, filed Apr. 3, 2009, which is acontinuation-in-part of U.S. patent application Ser. No. 12/266,477,filed on Nov. 6, 2008, which claims priority from ProvisionalApplication No. 60/985,855, filed on Nov. 6, 2007, the entire contentsof each of which are hereby incorporated by this express reference.

BACKGROUND

1. Technical Field

The disclosure relates generally to the synthesis of useful chemicalbyproducts and, more specifically, to the synthesis of ammonia usingnano-size metal catalyst particles.

2. Related Art

Ammonia synthesis is an important industrial process. Ammonia isproduced in huge quantities worldwide, for use in the fertilizerindustry, as a precursor for nitric acid and nitrates for the explosivesindustry, and as a raw material for various industrial chemicals.

Despite an energy production cost of about 35 to 50 GJ per ton ofammonia, the Haber-Bosch process is the most widespread ammoniamanufacturing process used today. The Haber-Bosch process was inventedin the early 1900s in Germany and is fundamental to modern chemicalengineering.

The Haber-Bosch process uses an iron catalyst to improve NH₃ yields.Being a transition metal with partially occupied d-bands, ironrepresents a surface suitable for adsorption and dissociation of N₂molecules. An example of a commonly used iron catalyst is reducedmagnetite ore (Fe₃O₄) enriched (“promoted”) with oxides of, for example,aluminum, potassium, calcium, magnesium, or silicon.

In the Haber-Bosch process, ammonia is synthesized using hydrogen (H₂)and nitrogen (N₂) gases according to the net reaction (N₂+3H₂→>2NH₃).The mechanism for iron-catalyzed ammonia synthesis is stated below infour dominant reaction steps, wherein “ads” denotes a species adsorbedon the iron catalyst and “g” denotes a gas phase species:

N₂(ads)→2N(ads)  (1)

H₂(ads)→2H(ads)  (2)

N(ads)+3H(ads)→NH₃(ads)  (3)

NH₃(ads)→NH₃(g)  (4)

The rate limiting step in the conversion of nitrogen and hydrogen intoammonia has been determined to be the adsorption and dissociation of thenitrogen on the catalyst surface. Thermodynamic equilibrium of thereaction is shifted towards ammonia product by high pressure and lowtemperature. However, in practice, both high pressures and temperaturesare used due to a sluggish reaction rate. Due to overall low reactionefficiency when hydrogen and nitrogen are first passed over the catalystbed, most ammonia production plants utilize multiple adiabaticallyheated catalyst beds with cooling between beds, typically with axial orradial flow. High pressure favors the adsorption process as well, but ata cost of increased operational and capital costs.

At pressures above 750 atm, there is an almost 100% conversion ofreactants to the ammonia product. Because there are difficultiesassociated with containing larger amounts of materials at this highpressure, lower pressures of about 150 to 250 atm are used industrially.By using a pressure of around 200 atm and a temperature of about 500°C., the yield of ammonia is about 10 to 20%, while costs and safetyconcerns in the plant and during operation of the plant are minimized.Nevertheless, due in part to high pressures used in the process, ammoniaproduction requires reactors with heavily-reinforced walls, piping andfittings, as well as a series of powerful compressors, all with highcapital cost. In addition, generation of those high pressures duringplant operation requires a large expenditure in energy.

In an effort to reduce the energy requirements of this process, theKellogg Advanced Ammonia Process (KAAP) was developed using a rutheniumcatalyst supported on carbon. The KAAP catalyst is reported to be 40%more active than the traditional iron catalysts. Use of this catalystallowed the reactor pressure to be reduced, but the high cost of theprecious metal ruthenium catalyst and the sensitivity of the catalyst toimpurities in the hydrogen feed stock have prevented widespread use forammonia synthesis. Other catalysts being studied include cobalt dopedwith ruthenium, but few encouraging results have been exhibited to date.Thus, after almost 90 years of ammonia synthesis, the Haber-Boschprocess remains the most commonly used ammonia synthesis mechanism.

For the last 100 years, iron-based catalysts have been used inindustrial ammonia synthesis. This catalyst is prepared by meltingmagnetite (Fe₃O₄) with a promoter compounds, for example potassium orcalcium, and solidifying. The resulting porous material is then crushedinto granules, generally in the size range of 1-10 millimeters. Activecatalyst is then produced by reduction of iron oxides with hydrogen andnitrogen gas mixture, to give porous iron, and unreduced promoteroxides. Approximately 50% of this catalyst is void volume.

Improvements to Haber-Bosch catalysts focus on the addition of promotersfor improved activity ammonia synthesis. U.S. Pat. Nos. 4,789,657 and3,951,862 describe processes of preparing a magnetite-based ammoniasynthesis catalyst via the melting of iron oxide with other compounds,such as Al₂O₃, K₂O, CaO, MgO, and SiO₂, and grinding into granules. U.S.Pat. No. 5,846,507 describes an iron composition having anon-stoichiometric oxide content and additional promoters, prepared bymelting. Suggestions of reducing the processing pressures have beenmade, but have not been achieved economically.

Non-ferrous metal oxides may also be incorporated into the granules. Forexample, U.S. Pat. No. 6,716,791 describes the addition of cobalt andtitanium oxides in a 0.1-3.0% weight ratio as additional promoters toaluminum, potassium, calcium, and magnesium. U.S. Pat. No. 3,653,831describes the addition of platinum to improve reaction efficiency,however given the expense of platinum this may not be feasible at largescales. Other promoters, such as cerium described in U.S. Pat. No.3,992,328 have also been shown to increase activity. Other improvementsinclude alternative catalysts, such as those described in U.S. Pat. Nos.4,163,775 and 4,179,407. These supported catalysts include ruthenium,rhodium, lanthanides and alloys.

Ideally, highly active ammonia catalysts can be used without significantchanges to the many existing ammonia plants that exist today; the bestcandidates would be a “drop in” solution for existing manufacturers.Retrofit and reconstruction of these plants could be costly should therebe a need to change design based on catalyst properties, such as spacevelocity. The best candidate catalyst would exhibit increased activity,have similar basic properties as compared to existing catalysts, andreduce operating costs. Non-ferrous catalysts in the above referencedprior art do not overcome all of these constraints because 1) catalystcost increases more than catalyst efficiency, 2) the catalyst may nothave the same properties that allow for seamless operation in existingammonia production plants, or 3) the catalyst may have high activity butdo not meet long term durability requirements.

SUMMARY

The invention described herein comprises the synthesis of ammonia byproviding core-shell iron/iron oxide nanoparticles on ferrous catalyststo improve catalytic activity while maintaining durability. In variousembodiments herein, systems and methods for the synthesis of ammonia aredisclosed that are capable of being used in both traditional and newammonia reactor bed designs. The function of the nano-size catalystparticles is improved by dispersing or separating the particles using asupport material, thereby reducing or eliminating sintering of adjacentparticles. The result are systems and methods that can operate at muchlower pressures than the Haber-Bosch process and that can maintaincatalysis efficiency over time.

In at least one embodiment of the present invention, methods ofsynthesizing ammonia are provided comprising reacting a supply ofnitrogen gas and hydrogen gas in the presence of nano-sized metalcatalyst particles disposed on a support material that is configured todisperse the catalyst particles, wherein the reaction proceeds at apressure less than about 500 atm., preferably less than about 200 atm.,and more preferably less than about 100 atm.

In certain embodiments and applications, the reaction proceeds costeffectively at pressures less than about 10 atm. In certain embodimentsof the inventive methods, at least a portion of the nano-sized metalcatalyst particles are selected from the group consisting of iron,cobalt, ruthenium, alloys thereof, and mixtures thereof. In certainembodiments, at least a portion of the nano-sized metal catalystparticles comprise a metal core and an oxide shell. In certainembodiments, the support material comprises a porous structure. Incertain embodiments, the support material comprises a matrix, tubes,granules, a honeycomb, or the like. In certain embodiments, the supportmaterial comprises magnetite or other ferrous materials, siliconnitride, silicon carbide, silicon dioxide, aluminum oxide, and/orcordierite, as examples. In certain embodiments, the support material isconfigured to separate the catalyst nano-particles. In certainembodiments, the support material further comprises promoter moleculeslocated adjacent the surface of the nano-sized metal catalyst particles.In certain embodiments, at least a portion of the promoter moleculescomprise one or more of the elements selected from the group consistingof Groups 1, 2, 6, 9, 13, 14 and the lanthanide series on the periodictable, including but not limited to aluminum, potassium, calcium,magnesium, and/or silicon. It should also be recognized that oxides,including core-shell oxides, of promoter material are also contemplatedfor promoting ammonia synthesis.

In one embodiment, at least some portion of the promoter comprisesnano-scale material to further enhance interaction between the promoterparticle and the nano-catalyst. Preferably, the promoter particle sizeis about 100 nanometers or smaller, although large nanoscale particlesare also suitable for enhanced promotion. In certain embodiments, thenanosized metal catalyst particles are disposed in a bed, with orwithout the support material.

In at least one embodiment of the present invention, an ammoniasynthesis reactor is provided, with nano-sized metal catalyst particlesdisposed within the reactor, wherein the catalyst particles may bedisposed on a support material that is configured to disperse thecatalyst particles. In certain embodiments of the reactor, at least aportion of the nano-sized metal catalyst particles are selected from thegroup consisting of iron, cobalt, ruthenium, alloys thereof, andmixtures thereof. In certain embodiments, at least a portion of thenano-sized metal catalyst particles comprise a metal core and an oxideshell. In certain embodiments, the support material comprises a porousstructure. In certain embodiments, the support material comprises amatrix, tubes, granules, a honeycomb, or the like. In certainembodiments, the support material comprises magnetite or other ferrousmaterials, silicon nitride, silicon carbide, silicon dioxide, aluminumoxide, or cordierite, by way of example. In certain embodiments, thesupport material is configured to separate the catalyst particles.

The reactor further comprises at least one inlet for providing hydrogengas and nitrogen gas to the nano-sized metal catalyst particles and atleast one outlet for removing ammonia gas generated in the presence ofthe nano-sized metal catalyst particles. The reactor is configured tooperate at a pressure less than about 500 atm., preferably less thanabout 200 atm., and more preferably less than about 100 atm. In certainembodiments, the reactor is a plug flow reactor, a packed bed reactor,an adiabatic reactor, or an isothermal reactor. In certain embodiments,the nano-sized metal catalyst particles are disposed in a packed bedwithin the reactor. In certain embodiments, the support material furthercomprises promoter molecules located adjacent the surface of thenano-sized metal catalyst particles. In certain embodiments, at least aportion of the promoter molecules are selected from the group consistingof aluminum, potassium, calcium, magnesium, and silicon.

In at least one embodiment of the present invention, a NO_(x)remediation system is provided that comprises a hydrogen gas supply anda nitrogen gas supply. The system further comprises a reactor in fluidcommunication with the hydrogen gas supply and the nitrogen gas supplycomprising nano-sized metal catalyst particles, wherein the nano-sizedmetal catalyst particles are disposed on a support material that isconfigured to disperse the catalyst particles, and wherein the reactoris configured to generate ammonia gas at a pressure less than about 500atm., preferably less than about 200 atm., and more preferably less thanabout 100 atm. The system further comprises an exhaust supply configuredto provide a gas stream comprising NO emissions and a selectivecatalytic reduction (SCR) system in fluid communication with the reactorand the exhaust supply, wherein the SCR system is configured tofacilitate the reaction of the ammonia gas and the NO emissions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a reactor comprising a bed of nano-sized metalcatalyst particles.

FIG. 2 is an SEM of nano-sized ferrous catalyst particles with an oxidelayer.

The features mentioned above in the summary, along with other featuresof the inventions disclosed herein, are described below with referenceto the drawings. The illustrated embodiments in the figures listed beloware intended to illustrate; but not to limit, the inventions.

DETAILED DESCRIPTION

In various embodiments, systems and methods of ammonia production areprovided. Referring first FIG. 1, an example system 10 is showncomprising at least one reactor 12. In preferred embodiments, thereactor 12 comprises a plug flow reactor. One or more alternativereactors can be used instead of or in conjunction with a plug flowreactor, for example, packed bed, adiabatic, and/or iso-thermalreactors. As an example, one or more reactors can be connected inseries.

N₂ gas 3 and H₂ gas 5 are introduced into the at least one reactor 12.The gases pass through a bed 14 of supported nano-sized metal catalystparticles disposed within the at least one reactor 12. A stream of NH₃gas 17 exits the at least one reactor 12. In certain embodiments, thestream of NH₃ gas 17 can be collected in a reservoir of water (notshown) after suitable cooling, to take advantage of the extensivesolubility of ammonia in water.

In certain embodiments, the supported nano-sized metal catalystparticles are disposed on the walls of the at least one reactor 14. Incertain embodiments, the supported nano-sized metal catalyst particlesare disposed within or on channels in the reactor 14. In certainembodiments, the supported nano-sized metal catalyst particles are piledin a packed bed configuration within the reactor. Alternativeconfigurations for arranging the supported particles within the at leastone reactor 14 can also be used. As used herein, “bed” 14 is used torefer to any suitable arrangement of supported nano-sized metal catalystparticles within the at least one reactor 14 and is not intended to belimited to a packed bed configuration.

The N₂ gas 3 and H₂ gas 5 are introduced into the at least one reactor12 at a pressure below about 200 atm, preferably below about 100 atm,and more preferably between about 1 atm and 20 atm (e.g., between about3 and 10 atm). Additional examples of pressures which have beendemonstrated to be suitable for ammonia synthesis are about 4 atm andabout 7 atm. In certain embodiments, the gases are heated totemperatures between about 200° C. and 600° C., and preferably betweenabout 400° C. and 450° C. In certain embodiments, the gases 3, 5 areheated before entering the bed 14. In certain embodiments, the gases areheated inside the bed 14. In certain embodiments, the molar ratio of N₂gas 3 to H₂ gas 5 introduced into the reactor 12 is about 1:10, 1:5,1:2, or 1:1. Preferably, the molar ratio of N₂ gas 3 to H₂ gas 5 isabout 1:3.

In certain embodiments, the N₂ gas 3 can be removed from compressed airusing an oxygen exclusion membrane. It is desirable to remove oxygen gasfrom the N₂ gas 3 feed because oxygen can reduce the efficiency of thereactions described above (e.g., by a side reaction to form water). Incertain embodiments, the H₂ gas 5 can be obtained from reformed naturalgas. In preferred embodiments, the H₂ gas 5 is provided by electrolysisof water.

Nano-sized metal catalyst particles as used herein refer to metalnanoparticles, metal alloy nanoparticles, nanoparticles having a metalor metal alloy core and an oxide shell, or mixtures thereof. Theparticles are preferably generally spherical, as shown in FIG. 2.Preferably the individual nanoparticles have a diameter less than about50 nm, more preferably between about 15 and 25 nm, and most preferablybetween about 1 and 15 nm. These particles can be produced by vaporcondensation in a vacuum chamber. A preferable vapor condensationprocess yielding highly uniform metal nanoparticles is described in U.S.Pat. No. 7,282,167 to Carpenter, which is hereby expressly incorporatedby reference in its entirety.

The nano-sized metal catalyst particles are disposed on a supportmaterial configured to disperse or separate the particles. It wassurprisingly discovered that a reactor 12 comprising a packed bed ofunsupported nano-sized metal catalyst particles nanoparticles tended tolose catalysis efficiency over time. At high temperatures, thenanoparticles sintered with adjacent nanoparticles, reducing the overallarea available for reaction on the particles' surfaces. The reduction ofsurface area due to temperature-induced sintering resulted in a loss ofcatalytic activity over time.

Experiments confirmed that sintering could be minimized and catalysisefficiency could be maintained by disposing the nano-sized metalcatalyst particles on a support material, thereby dispersing orseparating adjacent nanoparticles. Suitable structures for the supportmaterial include, but are not limited to, silicon nitride, siliconcarbide, silicon dioxide (silica), and aluminum oxide (alumina)matrices, granules, or tubes. An example of a suitable support materialis silica or alumina granules about 30 microns in diameter or Si₃N₄microtubes. Another example of a suitable support material is acordierite honeycomb. In certain embodiments, a porous material (e.g.,porous granules) can be used.

In certain embodiments, the support material can further comprisepromoter molecules disposed on or near the surface of the supportmaterial that contact, and in certain embodiments, are fused to theouter surface of the catalyst particles. Examples of suitable promotermolecules include, but are not limited to, aluminum, potassium, calcium,magnesium, and silicon. Promoter molecules can advantageously increasethe catalytic activity of nitrogen absorption and reaction with hydrogenduring ammonia synthesis by facilitating electron transfer.

In preferred embodiments, the nano-sized metal catalyst particlescomprise nano-sized ferrous (iron or iron alloy) catalyst particles.Other suitable metals can include cobalt, ruthenium, and alloys thereof.Mixtures of suitable metal catalyst particles can also be used incertain embodiments. For example, certain embodiments can comprise amixture of nano-sized iron and cobalt catalyst particles, a mixture ofcobalt and ruthenium catalyst particles, a mixture of iron and rutheniumcatalyst particles, or a mixture of iron, cobalt, and ruthenium catalystparticles.

As described above, in certain embodiments, at least a portion of thenano-sized catalyst particles have a metal or metal oxide core and anoxide shell. In preferred embodiments, the nano-sized ferrous catalystparticles comprise an iron or iron alloy core and an oxide shell. Anoxide shell can advantageously provide means for stabilizing the metalor metal oxide core. Preferably, the oxide shell has a shell thicknessbetween about 0.5 and 25 nm, more preferably between about 0.5 and 10nm, and most preferably between about 0.5 and 1.5 nm. Examples ofnano-sized ferrous catalyst particles comprising an oxide coatingthickness between about 0.5 and 1.5 nm are shown in FIG. 2. Theseparticles can be produced by vapor condensation in a vacuum chamber, andthe oxide layer thickness can be controlled by introduction of air oroxygen into the chamber as the particles are formed.

In certain embodiments, NO_(x) remediation systems are provided. Thesesystems can be integrated, for example, with internal combustionengines. In at least one embodiment, a vehicle comprising an on-boardNO_(x) remediation system is provided. The NO_(x) remediation systemsdisclosed herein advantageously reduce or eliminate NO_(x) emissionsfrom internal combustion engines by introducing ammonia or urea (whichis produced by reaction of ammonia and carbon dioxide) into the exhauststream.

An example NO_(x) remediation system comprises a reactor as describedabove. H₂ and N₂ gases are passed to the reactor, which comprises a bedof supported nano-sized metal catalyst particles. As described above, H₂gas can be produced by an electrolyzer system. In certain embodiments inwhich a NO_(x) remediation system is onboard a vehicle, the electrolyzeris powered by the vehicle's battery and/or engine alternator. N₂ gas canbe obtained by processing compressed air (e.g., from the brake system)through an oxygen exclusion filter. A stream of NH₃ is produced by thereactor.

The NH₃ stream is combined with exhaust from the internal combustionengine and directed into a selective catalytic reduction (SCR) catalystand filter. Preferably, the SCR catalyst comprises supported zeolitesand nano-sized metal catalyst particles such as nano-sized vanadium orvanadium alloy catalyst particles. In certain embodiments, the SCRcatalyst operates at a temperature between about 200° C. and 800° C. andmore preferably between about 400° C. and 600° C.

To determine how much NH₃ is required for NO_(x) reduction, in certainembodiments there is provided an electronic controller that uses theengine RPM and manifold pressure along with data from a NO_(x) sensor onthe exhaust of the SCR catalyst to increase or decrease the amount ofcurrent to the electrolyzer controlling the hydrogen input to the lowpressure ammonia generator. The larger the amount of ammonia generated,the greater the overall NO_(x) reduction in the exhaust stream.

Example 1

Synthesis of NH₃ was performed over a bed of nano-sized ferrous catalystparticles, manufactured using the vapor condensation process describedin U.S. Pat. No. 7,282,167 to Carpenter, and supported with siliconnitride tubes. The nano-sized ferrous catalyst particles comprised anoxide coating between about 0.5 and 1.5 nanometer thickness. Theparticles had average diameters from 15 to 25 nanometers.

The supported nano-sized ferrous catalyst particles were piled in apacked bed configuration within a plug flow reactor system. Hydrogen andnitrogen gases were introduced into to plug flow reactor system asdescribed above at pressures between about 10 atm and 20 atm and atemperature of about 450° C.

Ammonia was detected and alkalinity tests conducted with pH paperyielded a pH of 11, typical of ammoniacal solutions in water. Theexperiment established the production of ammonia from hydrogen andnitrogen at vastly reduced pressures, as compared to industrialprocesses for ammonia synthesis, by a factor of 15 to 30. The kineticrate of the adsorption and disassociation of the nitrogen and hydrogenwas increased by as much as three orders of magnitude.

Example 2

Synthesis of ammonia was performed over a bed of nano-sized ferrouscatalyst particles, manufactured using the vapor condensation processdescribed in U.S. Pat. No. 7,282,167 to Carpenter, and supported onSG9801R promoted iron from BASF. The nano-sized ferrous catalystparticles comprised an oxide coating between about 0.5 and 1.5 nanometerthickness rendering them air safe for mixing. The particles had anaverage diameter from 15 to 30 nanometers. The nano-sized iron and ironsupport particles were blended for 2 minutes at 20G with an acousticmixer to distribute the nano-sized particles onto the support ironparticles.

Supported nano-sized ferrous catalyst particles were piled in a packedbed configuration within a plug flow reactor system. The supportednano-sized iron particles were reduced in a stream of hydrogen gas at300° C. Hydrogen and nitrogen gasses were introduced into the plug flowreactor system as described above at pressures between about 5 atm and10 atm and a temperature of 350° C. to 450° C.

Ammonia production was quantified by bubbling the mixture of gassesflowing from the reactor through a measured amount of dilute sulfuricacid containing a phenolphthalein indicator and recording the timerequired to reach a pink end point. The experiment established theproduction of ammonia from hydrogen and nitrogen at vastly reducedpressures, as compared to industrial processes for ammonia synthesis, bya factor of 15 to 30. The kinetic rate of the adsorption anddisassociation of the nitrogen and hydrogen was increased by as much asthree orders of magnitude compared to conventional iron catalysts.

The foregoing description is that of preferred embodiments havingcertain features, aspects, and advantages. Various changes andmodifications also may be made to the above-described embodimentswithout departing from the spirit and scope of the inventions. Forexample, it is contemplated that nano-sized materials made fromprocesses other than the ones described in U.S. Pat. No. 7,282,167 toCarpenter would still achieve some or all of the advantages describedabove or inherent herein, including cost effective ammonia synthesis. Itis also contemplated that pressures well below prior art conventionalprocessing of 200 atmospheres can be achieved using the inventiveprocess herein.

1. A method of synthesizing ammonia comprising reacting a supply ofnitrogen gas in the presence of nano-sized metal catalyst particlesdisposed on a ferrous support and in the presence of a nano-sizedpromoter.
 2. The method of claim 1, wherein the metal catalyst comprisesan oxide layer.
 3. The method of claim 1, wherein the promoter comprisesmolecules from Groups 1, 2, 6, 9, 13, 14 and/or the lanthanide series onthe periodic table.
 4. An ammonia synthesis reactor comprising:nano-sized iron catalyst particles and a nano-sized promoter disposed ona ferrous support material within the reactor; at least one inletconfigured to introduce hydrogen gas and nitrogen gas to the nano-sizedmetal catalyst particles; and at least one outlet configured to removeammonia gas generated in the presence of the nano-sized metal catalystparticles, wherein the reactor is configured to operate at a pressureless than about 500 atm.
 5. The method of claim 4, wherein the metalcatalyst comprises an oxide layer.
 6. The method of claim 4, wherein thepromoter comprises molecules from Groups 1, 2, 6, 9, 13, 14 and/or thelanthanide series on the periodic table.